Kelso & Co, Global Ship Lease's (GSL) largest shareholder, plans to significantly reduce its position in the New York-listed boxship owner.

The US private equity firm expects to sell 4.5m common shares at $12.50 each through a previously announced secondary offering.

It has also given the offer's underwriters a 30-day option to buy 675,000 more shares, offering up a $64.7m